Regular refresh in Excel through C# add-in

Question

I am trying to get a random number in Excel that would get refreshed at a regular time interval, by using a C# add-in exposed through the Microsoft.Office.Interop.Excel library. The function called in Excel is named "GetRandomLive" with the interval as an argument and returns the random number and a timestamp.

To check whether my Excel application is correctly identified, I have added a line that writes on the dummy cell "A1" the timer event SignalTime.

In the test, I have set an interval equal to 10,000 ms. I have checked that the Excel "Calculation Options" are correctly set to "Automatic". However, if indeed the C# library correctly writes the SignalTime 10,000 ms apart on the Excel target cell, it does not always recalculate the sheet. I tried several things (as per the code below) that leave me puzzled:

1) _excel.CalculateFull();     // => Works
2) _excel.Calculate();         // => Does not work
3) _sheet.Calculate();         // => Does not work

Obviously, I would not like to have to resort to a full recalculation of the Excel app, as it could easily become an overkill (in the presence of potentially many other complex calculations on various sheets). Ideally, I would like to restrict the recalculation to the function calling sheet or, even better, to the function calling cell or range.

Here is the code that I have written. Any light would be welcome.

using System;
using System.Timers;
using System.Runtime.InteropServices;
using Excel = Microsoft.Office.Interop.Excel;

namespace Space
{
    public class RandomLive
    {
        private static Excel.Application _excel;
        private static Excel.Worksheet _sheet;
        private static Timer _timer;
        private static bool _first = true;
        private static object[,] _data;

        public RandomLive()
        {
            _excel = Marshal.GetActiveObject("Excel.Application") as Excel.Application;
        }

        public object[,] GetRandomLive(int interval)
        {
            _sheet = _excel.ActiveWorkbook.ActiveSheet;

            if (_first)
            {
                _timer = new Timer();
                _timer.Elapsed += (sender, args) => TimerElapsed(sender, args);
                _timer.AutoReset = true;
                _first = false;
            }
            _timer.Interval = interval;
            _timer.Enabled = true;
            _timer.Start();

            _data = GetRandom();
            return _data;
        }

        private static void TimerElapsed(object sender, ElapsedEventArgs args)
        {
            _data = GetRandom();
            try
            {
                _sheet.Cells[1, 1].Value = args.SignalTime;
                //_excel.CalculateFull();     // => Works
                //_excel.Calculate();         // => Does not work
                _sheet.Calculate();         // => Does not work
            }
            catch
            {
                return;
            } 
        }

        private static object[,] GetRandom()
        {
            _data = new object[2, 2];
            _data[0, 0] = "Random";
            _data[1, 0] = "Timestamp";

            Random random = new Random();
            double value = random.NextDouble();
            _data[0, 1] = value;
            _data[1, 1] = DateTime.Now;

            return _data;
        }
    }
}

10 seconds after the first function call, the event SignalTime refreshes, as expected

However, after a another batch of time, if indeed the SignalTime still refreshes, the function value does not